Typically, a subterranean formation penetrated by a well has a plurality of distinct zones or formations of interest. During production of fluids from the well, it usually is desirable to establish communication with only the zone or formations of interest such that stimulation treatments do not inadvertently flow into a non-productive zone or a zone of diminished interest. Selective stimulation (such as by hydraulic fracturing and acid stimulation) becomes pronounced as the life of the well declines and productivity of the well decreases.
Conventionally, selective stimulation was done by one or more perforating guns wherein the gun was conveyed on a wireline or tubing into the well and the gun was positioned adjacent to the zone and/or formation of interest and then selectively fired to perforate the zone and/or formation. The gun was then repositioned by the wireline to another zone or formation and the zone or formation of interest was then selectively perforated. This procedure was repeated until all of the zones and/or formations of interest were perforated; the perforating gun was then retrieved to the surface by means of the wireline. When fracturing was desired, the fracturing fluid was then pumped into the well under pressure exceeding the pressure at which the zone and/or formations would fracture. In order to prevent the fracturing fluid from flowing into zones having greater porosity and/or lower pressure, a mechanical device, such as a straddle packer, or plug or sand fill was first set in the well between the zone just fractured and the zone to be fractured to isolate the stimulated zone from further contact with the fracturing fluid. This procedure was repeated until all of the zones of interest were perforated and fractured.
Once the completion operation was finished, each plug had to be drilled out of or otherwise removed from the well to permit fluid to be produced to the surface through the well. The necessity of tripping in and out of the wellbore to perforate and stimulate each of the multiple zones and the use of such plugs to isolated previously treated zones and/or formations from further treatment fluid contact was both time consuming and expensive.
Various methods and assemblies have been reported for effectuating zonal isolation between intervals of the wellbore that did not depend on the removal of perforating equipment in and out of the well when completing multiple zones of interest. At the same time, such methods and assemblies isolated selected targeted productive intervals of the wellbore from non-productive intervals.
More recently, the use of isolation assemblies have been reported to provide zonal isolation and which allow for selected treatment of productive (or previously producing intervals) in multiple interval wellbores. For instance, U.S. Pat. No. 6,386,288 discloses a mechanical zonal isolation system which may be provided on the outside of the casing string (cemented to the wellbore) to permit an interval of the formation to be completed and stimulated and/or treated independent of the others. In this manner, selected intervals of the subterranean formation may be stimulated and/or treated. Such assemblies may include the use of flapper valve assemblies positioned between perforating gum assemblies.
See further, U.S. Pat. No. 7,575,062 which discloses an isolation assembly containing screen-wrapped sleeves and a plurality of swellable packers disposed in a liner and a tool within the liner for shilling openings fix the control of fluids from the wellbore.
Zonal isolation assemblies are expensive. When held in place by a cementitious slurry, such assemblies may only be removed from the wellbore by damaging or destroying the assembly. Alternative methods are further needed which will hold the casing in place in the wellbore.
In addition, alternatives have been sought for securing casing to the wellbore. Traditionally, cementitious slurries are used to cement the well pipe and casing to the wellbore. Typically, the slurry is pumped down the inside of the pipe or casing and back up the outside of the pipe or casing through the annular space between the exterior of the casing and the wellbore. The cement slurry is then allowed to set and harden to hold the casing in place. The use of conventional cementitious slurries is undesirable for use with zonal isolation assemblies since, in order to be removed from the well, damage to or destruction of the zonal isolation assembly becomes necessary.
Alternatives have also been sought for methods of re-fracturing a selectively stimulated formation having a plurality of distinct production zones. Re-fracturing is often desirable when a production zone within the well may not have been adequately fractured. This typically results in inadequate production from the production zone. Even if the formation was adequately fractured, the production zone may no longer be producing at adequate levels. Over an extended period of time, the production from a previously fractured horizontal wellbore may decrease below a minimum threshold level. One technique to increase the hydrocarbon production is the addition of new fractures within the subterranean formation.